(1) Field of the Invention
This invention relates to an apparatus having an automatic focusing (Af) function such as an AF camera, and more particularly to an automatic focusing device capable of accurately moving a focusing movable lens to a focused position and changing the moving speed of the focusing lens according to an amount of defocusing to the focused position during the focusing operation.
(2) Description of the Prior Art
Recently, there have been available some interchangeable-lens cameras having an AF function.
FIG. 1 is a block diagram outlining the general construction of an interchangeable-lens camera of this type. More specifically, FIG. 1 mainly shows components of the camera which concerns the AF mechanism, not components which are generally provided for a camera and a lens.
In FIG. 1, reference numeral 11 designates a camera body; and 31, a photographical lens detachably mounted on the camera body 11. The camera body 11 and the lens 31 are mechanically connected to each other through a clutch 13 on the body side and a clutch 33 on the lens side, and are electrically connected to each other through an electrical contact group 15 on the body side and an electrical contact group 35 on the lens side.
The photographing lens 31 comprises: a lens system 39 including a focusing lens 37 which is movable along the optical axis; a drive force transmitting mechanism 41 for transmitting a drive force from a drive source (described later) to the lens 37 to move the latter 37 to a focused position to an object; and a lens ROM 43 for storing the aperture data of the photographing lens, the position data of the movable lens 37 and the like.
On the other hand, the camera body 11 comprises: an image pickup section 17 having, for instance, a CCD (charge-coupled device) sensor to receive a part of the light beam passed through the photographing lens 31 thereby to form the image of the object. The camera body 11 further comprises: a control section 19 having a variety of functions such as a function of calculating an amount of defocusing D (including a direction of defocusing) indicating the amount of deviation from the focusing position in response to the output signals of the image pickup section 17 and a function of determining the direction of movement of the lens for focusing the lens 37 on an object. The camera body 11 further comprises: a drive mechanism 25 including an electric motor 21 for driving the movable lens 37 in the photographing lens 31, and an encoder 23 for controlling the number of revolutions of the motor 21. The drive force of the drive mechanism 25 is transmitted through the clutch mechanism 13 and 33 and the drive force transmitting mechanism 41 to move the movable lens 37.
An amount of drive P moving the movable lens 37 to the focused position can be determined in terms of a number of rotation of the motor 21, that is, it can be set according to the pulse count number of the pulse signal from the encoder 23 which detects the number of revolutions of the motor 21. In the conventional AF camera, the amount of defocusing D is calculated by the control section 19, and the pulse count value P corresponding to the amount of defocusing D is calculated according to the following equation (1): EQU P=K.multidot.D (1)
where K is the lens movement conversion coefficient.
The coefficient K is predetermined so as to obtain a pulse count number P allowing the movable lens 37 to move to the position where the lens is positively focused on the object according to the amount of defocusing D, and each photographing lens has its own coefficient K. The coefficient K is stored in the lens ROM 43 of the photographing lens. In the case where the photographing leans is a zoom lens, a plurality of coefficients are stored in the lens ROM.
The movable lens 37 is continuously moved until the number of pulses of the output signal from the encoder 23 reaches the pulse count value P calculated according to equation (1).
In the case where it is required to focus the lens on an object with higher accuracy, the lens is driven intermittently; that is, the lens is moved by small intervals, and every time the lens is moved, a focusing pulse count value is newly obtained.
A method for obtaining an amount of defocusing with respect to an object to be photographed will be described hereinafter.
First, the CCD sensor of the image pickup section 17 receives light from the object to provide defocusing data including an amount of defocusing and a direction of defocusing according to the correlation method. However, in the case where the present position of the movable lens is far from the focused position for the object, it is sometimes impossible to determine an amount of defocusing effectively in calculating the amount of drive to the focused position. That is, sometimes the amount of defocusing cannot be obtained or the amount of defocusing includes an error. In this case, heretofore the following method is employed: The movable lens, after being moved for a distance in one direction at a constant speed, is stopped, and an amount of defocusing is obtained as described above. This operation is repeatedly carried out until the amount of defocusing thus obtained is effectively determined. In the case where the lens is moved in one direction, and no effective amount of defocusing is obtained although the lens has reached the end point in the one direction, the lens is moved in the opposite direction, and the above-described series of operations are carried out until the amount of defocusing is determined.
However, when the movable lens is moved at a stretch according to the amount of drive of the movable lens obtained according the amount of defocusing D, sometimes the resultant focal accuracy of the lens is lower than required.
This difficulty is significant when the amount of defocusing is large. That is, when the lens is greatly defocused from an object, the amount of defocusing obtained is often incorrect, including an error. Therefore, the amount of drive of the lens to the focused position obtained from the amount of defocusing includes an error, as a result of which the lens is not satisfactorily focused on the object. Generally, in order to move the movable lens 37 to the focused position, the motor 21 is driven continuously in a DC control mode. In this case, the movable lens 37 further moves due to its inertia. Accordingly, the motor 21 has to be stopped immediately before a pulse count value of a pulse signal from the encoder 23 becomes equal to the number of pulses P which is obtained according to the above equation (1).
However, the above described driving method in the DC control mode is disadvantageous in that it is somewhat difficult to stop the movable lens 31 at the focused position with high accuracy.
Further, the above cyclic operation for obtaining an amount of defocusing which is effective for the focusing operation is disadvantageous in that, since the integration of the CCD sensor is carried out after the movable lens is positioned within a suitable range from the focused position, it takes a relatively long period of time to obtain the defocusing amount thereby requiring a long period of time for focusing. This drawback may cause the photographer to lose his chance to take a desired photograph.
In order to overcome the drawbacks, it has been proposed by the present inventors that, after the pulse count value which represents the remaining amount of drive becomes smaller than a predetermined value, the driving mode of the motor 21 is changed to a PWM (Pulse Width Modulation) mode where the motor 21 is driven intermittently to let the lens 37 approach the focused position gradually. In this case, however, where the defocusing amount is extremely small, the movable lens is driven in the PWM control mode from the start of the movement. As is well known in the art, an ordinary motor requires a large amount of drive power and such a PWM control mode is not suitable to obtain the large amount of drive power. Therefore, this also results in a problem such that the lens may not be started and moved to the focused position quickly.